JP2015519705A - Electrothermal planar cathode - Google Patents

Abstract

An electrothermal planar cathode for a miniature X-ray tube can be a spiral design that is laser cut from a thin tantalum alloy ribbon foil (116) with particle stabilization properties. The ribbon is attached to the substrate of the aluminum nitride substrate (110) in such a way as to give the bare ribbon a minimum tension and then machined into a spiral pattern (118). The helical pattern can be optimized with respect to electrical, thermal and emission characteristics. [Selection] Figure 1C

Description

  An X-ray tube is a vacuum tube that generates X-rays. The x-ray tube includes a cathode that emits electrons into a vacuum and an anode that collects the electrons. A high voltage power source is connected between the cathode and the anode to accelerate the electrons. Some devices require X-ray tubes that require very high resolution images and that can form very small focus sizes.

  One type of cathode, like the bulb filament, contains a tungsten filament that is spirally wound. The problem with wound filaments is that electrons are emitted from surfaces that are not perpendicular to the accelerating field. This makes it very difficult to focus the electrons on a compact spot on the X-ray target.

  Electrothermal planar cathodes for small x-ray tubes include spiral designs that are laser cut from foils such as thin tantalum alloy ribbon foils, which can have particle stabilization properties. The ribbon is brazed to a substrate, such as an aluminum nitride substrate, in such a way as to give the bare ribbon a minimum tension, and then machined into a geometric pattern, eg, a spiral. This prevents twisting of the planar pattern due to the cutting process or handling and mounting. Optionally, the helical pattern can be optimized for electrical and thermal properties. The resulting cathode assembly is mounted on the header and mechanically and electrically connected to other components of the x-ray tube.

FIG. 1A shows a planar cathode structure before cutting. FIG. 1B shows the planar electrode structure after laser cutting. FIG. 1C shows the implemented planar cathode structure. FIG. 2 shows a process flow chart of the planar cathode shown in FIGS. 1A and 1B.

  Electrothermal planar cathodes for small x-ray tubes include a spiral design that is laser cut from a thin tantalum alloy ribbon foil (having particle stabilization properties). The ribbon is brazed to the aluminum nitride substrate in such a way as to give the bare ribbon a minimum tension and then machined into a geometric pattern, for example a spiral. This prevents twisting of the planar pattern due to the cutting process or handling and mounting. The helical pattern can be optimized for electrical and thermal properties. The resulting cathode assembly is mounted on a header (sometimes referred to as a “first substrate”) and mechanically and electrically connected to other components of the x-ray tube. The remaining tantalum tape outside the cathode helix forms an equipotential surface, which helps form an electron beam that is very parallel and easy to focus.

  Certain implementations solve this structural vulnerability problem by mounting the foil on the substrate prior to machining. The use of particle-stabilized foils or particle-stabilized metals such as particle-stabilized tantalum is important. This is because the grain growth induced when the cathode works at the operating temperature can cause mechanical twisting or distortion. This twist pulls the helix away from the plane of the tantalum ribbon.

  FIG. 1A shows the structure of a flat cathode before cutting. The AlN substrate 110 includes an optional alignment mechanism 112 and a hole 114. A tantalum ribbon 116 that is brazed to the AlN substrate 110 is mounted over the hole 114. The ribbon (eg, tantalum) slightly overlaps the substrate, allowing the substrate to absorb stray emission current during operation. The hole 114 is illustratively shown larger than required.

  FIG. 1B shows a planar cathode structure after laser cutting. A spiral cut 118 is introduced. The spiral cut inlets and outlets are rounded or curved to minimize sharp corners, thus reducing stray emission currents. In this embodiment, the spiral cut entry and exit are exaggerated to better illustrate the corner minimization.

  In this illustrative embodiment, substrate 110 is made of aluminum nitride (AlN).

  Although this embodiment shows a geometric pattern of tantalum ribbon 116 floating over an opening 114 in substrate 110 (specifically, a spiral cut is shown), the opening is optional. It is. It is necessary to provide thermal isolation between the geometric pattern and the substrate 110. Illustratively, thermal isolation can be achieved by opening, cavities, or by floating the pattern over the substrate 110 such that a gap is created.

  FIG. 1C shows a planar cathode and lens assembly 120 mounted on a typical header 130.

  FIG. 2 is a process flow chart of the planar cathode shown in FIGS. 1A and 1B. In step 12, the tantalum foil is brazed to the AlN substrate. This brazing can be done by forming a laminate with foil, using an active brazing material on an AlN substrate, or by metallizing the substrate and using a normal brazing process. This can be done. In step 14, the helical pattern is laser cut or etched. The resulting cathode can be handled without damaging the spiral pattern thanks to the substrate. An optional alignment mechanism is added during substrate manufacture. This is because machining it after brazing or cutting can damage the helix. In this process, before cutting the helix, the alignment mechanism is used to calibrate the position so that the helix is in the middle of the alignment mechanism. At step 18, the cathode assembly is attached to the header 130 via an alignment mechanism to provide electrical connection and mechanical alignment between the cathode and other electro-optic components.

  In the illustrative example, the tantalum ribbon is brazed to the AlN substrate. Because they have a similar coefficient of thermal expansion. When the cathode is cut it remains flat.

  This concept can be extended to other materials that do not evaporate or twist over time. Foil materials include, but are not limited to, tungsten rhenium, triated tungsten, tungsten alloys, hafnium, and other tantalum based materials that exhibit an electron work function of less than 6 eV. A coating can be added to the helix to reduce the work function of the helix, thereby allowing the use of a variety of helix materials and reducing the temperature and power required to form sufficient electron flux.

Claims (18)

A first substrate;
A laminate of foil and a second substrate, wherein the foil and the second substrate have matching thermal expansion coefficients, and the laminate is attached to the first substrate When,
A planar cathode comprising:
The foil is formed in a predetermined geometric pattern, the foil having performance parameters selected from the group comprising area, voltage, current, power, and electron emission;
A planar cathode, wherein the geometric pattern and the first substrate are thermally separated. The first substrate further includes an alignment mechanism,
The planar cathode of claim 1, wherein the alignment mechanism is selected from the group comprising a hole, a mechanical mechanism, and an optical mechanism.   The planar cathode according to claim 1, wherein the laminate of the foil and the second substrate is a tantalum foil brazed to an AlN substrate.   The planar cathode of claim 1, wherein the predetermined geometric pattern is a spiral cut on the foil.   The planar cathode of claim 4, wherein the helical cut includes a curved inlet and a curved outlet.   The planar cathode of claim 1, wherein the foil is selected from the group comprising tungsten rhenium, triated tungsten, tungsten alloy, hafnium, and a tantalum based material having a work function of less than 6 eV.   The planar cathode of claim 1, wherein the foil is coated to exhibit an electron work function of less than 6 ev. Brazing the foil to the substrate to form a laminate;
Forming the foil of the laminate in a predetermined geometric pattern;
Mounting the laminate on a header;
A method for producing a flat cathode comprising:   The method of claim 8, wherein the predetermined geometric pattern is helix.   The method of claim 9, wherein the helix includes a curved inlet and a curved outlet.   The foil is selected from the group comprising tungsten rhenium, triated tungsten, tungsten alloys, and other refractory material based thermionic emission materials, or is a low work function emission coated cathode. The method of claim 8.   9. The method of claim 8, wherein the foil is selected from the group comprising tungsten rhenium, triated tungsten, tungsten alloy, hafnium, and a tantalum based material having a work function less than 6 eV.   9. The method of claim 8, comprising applying a coating to the foil to exhibit an electronic work function of less than 6 eV.   The method of claim 8, wherein the foil is brazed to an AlN substrate.   The planar cathode of claim 1, wherein the foil comprises a particle-stabilized foil.   The planar cathode of claim 1, wherein the foil comprises a particle stabilized tantalum foil.   The planar cathode of claim 16, wherein the substrate comprises an AlN substrate.   The planar cathode of claim 1, wherein the foil is selected from tungsten rhenium, triated tungsten, tungsten alloy, hafnium, and a tantalum-based material having a work function of less than 6 eV.

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